Aluminum-silicon alloy



Patented Nov. 7, 1933 UNITED STATES PATENT OFFICE ALUIHINUM- SILICONALLOY No Drawing. Application January 30, 1936 Serial No. 424,726,

4 Claims.

The invention relates to aluminum-silicon casting alloys and it isparticularly concerned with new alloy compositions having improvedcasting properties.

5 The use of aluminum alloys in cast articles has received considerableimpetus in recent years because of the adaptability of these alloys tovarious conditions of casting practice and their desirable combinationof strength, lightness and excellent appearance in the cast condition.Among those aluminum alloys which have obtained wide-spread applicationin the casting art the aluminum-silicon alloys containing substantialamounts of silicon are, in importance and extensive use, second only toaluminum base alloys containing copper.

The physical properties of aluminum-silicon alloys, including theirresistance to the corrosive agencies commonly encountered, areespecially advantageous for small and intricate castings such as motorparts and miscellaneous fittings, and for east articles such as cookingutensils, as well as comparatively large structural shapes of variousdesign. The aluminum silicon alloys generally used for these purposescontain between about 3 to 15 per cent of silicon together with a smallamount of iron as impurity, and sometimes copper, nickel, zinc,magnesium or similar alloying elements added in varying amounts toimpart certain particular and beneficial properties. Among alloys ofthis type the aluminum-silicon-copper alloys are, for example, wellknown. Alloys containing somewhat smaller amounts than 3 per cent ofsilicon together with copper, nickel, etc., are used to some extent forcasting purposes.

When, however, aluminum-silicon alloys are cast there are often formedsmall depressions or holes on the surface of the casting which areusually known as shrinks or draws. These are apparently due to shrinkageat the boundaries of the grains within the metal. In any casting theseshrinks are undesirable, and in many cases they are so numerous andtheir effect on the strength, appearance and other properties of thecasting is so objectionable as to preclude the use of the casting forits intended purpose, thus necessitating numerous rejections of castingswhich otherwise are perfectly good.

The present invention is predicated upon my discovery that this serioustendency of aluminumsilicon alloy to form shrinks or draws onsolidification may be inhibited, in some cases completely, by thepresence of a small amount of titanium in the alloy, and I have foundthat the addition will allow the use of aluminumsilicon alloys forcastings which had not heretofore been considered possible with thesealloys. Furthermore, the addition of the titanium to these alloys doesnot appear to decrease their natural strength.

One of the desirable features of my invention is the small amount oftitanium required to eliminate these shrinks or draws. I have determinedthat the amount of alloyed titanium which will produce the mostbeneficial results may vary slightly according to the particularaluminum-silicon alloys with which it is used but will usually fallwithin the range of 0.1 to 0.5 per cent. Ordinarily it is not necessaryto have present in the alloy more than about 0.1 to 0.2 per cent oftitanium. In some cases the titanium content may be even less. Greateramounts ithan 0.5 per cent may be added but the segregation caused bythe high melting titanium constituent which is formed when titanium isadded to aluminum makes it inadvisable to exceed this concentration.

In any casting operation there are, as is well known, a number ofvariables, many of them 8 elusive and diflicult to control, which mustbe favorably adjusted before good castings can be obtained, but even themost careful observance of these well known conditions is insufficient,in the case of aluminum-silicon alloys, to secure uniformly goodcastings in respect to freedom from shrinks and'- draws, whereas with myaluminum-silicon-titanium alloy this important uni" formity of resultsis readily obtainable when conditions which are known to give poorresults are avoided.

In casting the alloy the best results will be obtained if thetemperature of. the molten metal be kept as low as possible consistentwith proper pouring and the time of holding the metal molten beforepouring the casting be not too long. However, in the usual foundrypractice it is at times necessary to pour at a higher temperaturebecause of the casting design and section thickness or to hold the metalmolten for a considerable time before pouring. This latter practice willhave an injurious tendency which I have found to be offset by the use oftitanium as described in this specification.

The manner in which titanium is added to the alloy is not important andin so far as the final result is concerned, any of the well knownmethods of addition may be used. However, the usualprecautionsemployedwith good melting and casting practice should beobserved. It has been found advantageous to add the titanium in thecourse of. the electrolytic production of the aluminum rather than toadd it to the final aluminum-silicon alloy. A titanium-bearing ore, forexample titanium oxide, or alumina containing titanium oxide, may beadded to the fluoride bath in the electrolytic pot, so that the titaniumcompound will, during the reduction process, be reduced to metallictitanium which will alloy with the aluminum. Silicon may then be addedto this titanium-bearing aluminum in any convenient form, such as asilicon-rich aluminumsilicon alloy, or as metallic silicon. The aluminumwhich is used in making these alloys may be of any commercial grade ormay be aluminum of the highest purity. Usually, however, it is advantageous for economic reasons to use aluminum of commercial grade,though containing small amounts of iron and other impurities.

Castings of the aluminum-silicon-titanium a1- loy may be made from theuntreated molten alloy and also from the alloy which has, beforecasting, been treated to modify or improve its internal structure.Treatments for such purposes are well known and may consist, forexample, in the addition of metallic sodium or potassium or a compoundor compounds thereof to the molten alloy directly before casting. Theultimate eifect of this treatment is to produce a casting havingsubstantially higher strength and ductility than is found in theso-called unmodified alloy.

The importance of the elimination of the shrinkage defect inaluminum-silicon alloy may be illustrated by the case of a large foundrywhich makes a variety of cast aluminum cooking utensils of more or lessintricate design. This foundry had encountered the usual difficulty inobtaining good castings of aluminum-silicon alloys containing 5 per centof silicon, particularly in some of the more intricate castings, theselatter always showing so large a number of shrinks or draws on theirsurface as to be unsuitablefor use. Because of this it was foundnecessary to substitute for the aluminum-silicon alloy analuminum-copper alloy containing 5 per cent of copper which, althoughnot entirely overcoming the difficulty, made posisble the production ofabetter casting. This substitution, however, was objectionable because ofthe inferior corrosionresisting properties of the aluminum-copper alloy.But with aluminum-silicon alloy containing 5 per cent of silicon towhich there had been added, in accordance with my discovery, 0.1 percent of titanium, the trouble was obviated and castings were regularlyproduced of a quality which had theretofore been considered impossible.This specific instance is given only by way of illustration.

The term aluminum-silicon-titanium alloy is used in the appended claimsas a generic expression to include the following alloys and none other:(a) Alloys consisting entirely of aluminum, silicon, and titanium; and(b) alloys consisting of aluminum, silicon, titanium, and one or moremetals of the class composed of copper, nickel, zinc, magnesium and thelike; the titanium in each case serving to substantially inhibit theformation of surface shrinks or draws in casting the alloy.

It is to be understood that the invention is not limited to the specificdetails herein described but may be carried out in other ways withoutdeparture from its spirit.

I claim- 1. An aluminum-silicon-titanium casting alloy composed chieflyof aluminum, having about 3 to 15 per cent silicon and about 0.1 to 0.5per cent titanium, the latter element serving to substantially inhibitthe formation of surface shrinks or draws when the alloy is cast.

2. An aluminum-silicon-titanium casting alloy composed chiefly ofaluminum, having about 3 to 15 per cent silicon and about 0.1 to 0.2 percent titanium, the latter element serving to substantially inhibit theformation of surface shrinks or draws when the alloy is cast.

3. An aluminum base casting alloy consisting of silicon between about 3and 15 per cent, titanium between about 0.1 and 0.5 per cent, and thebalance aluminum, characterized, in the cast condition, by substantialfreedom from surface shrinks or draws.

4. An aluminum base casting alloy consisting of silicon between about 3and 15 per cent, titanium from about 0.1 to about 0.2 per cent, and theremainder aluminum, characterized, in the cast condition, by substantialfreedom from surface shrinks and draws.

THERON D. STAY.

